This didn't make sense to me

Rating: Null/10 (19 July 2003)

Maybe someone can explain this to me. It is said that there are some cases of infected people in New York and Paris. The disease takes 10 to 20 seconds to develop once a person is infected. Given that, why would any infected people be allowed on airplanes to spread the disease? It's obvious they are infected. And if somehow one did get on an airplane, the entire plane full of people would be infected before landing. Why would that plane be allowed to land?